JPH051595B2 - - Google Patents

Description

[Detailed description of the invention]

[Technical Field of the Invention] The present invention relates to a lithium secondary battery, and more particularly to a lithium secondary battery having a good electrolyte. [Background of the invention] Lithium batteries have a high standard unipolar potential, -3.03V based on standard hydrogen electrodes, and have extremely strong reducing power.
Also, since the atomic weight is small at 6.941, the capacity density per weight is high at 3.86Ah/g. For this reason, batteries that use lithium as the negative electrode active material (hereinafter referred to as lithium batteries) are being researched as small-sized batteries with high energy density, and batteries that use manganese dioxide, graphite fluoride, etc. as the positive electrode active material are already commercially available. ing. However, these commercially available lithium batteries are primary batteries, and at present, a rechargeable and dischargeable lithium secondary battery for practical use has not been realized. If lithium batteries can be recharged while taking advantage of the discharge characteristics of high energy density, a battery with extremely superior characteristics compared to conventional battery systems will be realized, and it will be useful for portable electronic devices and other devices. The effect on industrial associations is high. In order to secondaryize lithium batteries, there are many issues that need to be resolved, such as the selection of positive electrode active materials and battery construction methods. In particular, the selection of electrolyte is an important issue. Although it is preferable to use a non-aqueous electrolyte for a lithium secondary battery that operates at room temperature for practical detection,
The drawback is that the conductivity of the electrolyte is one or two orders of magnitude lower than that of the aqueous solution systems used in conventional battery systems. For this reason, improving the conductivity of the electrolyte is essential for battery discharge utilization factories. At the same time, in order to apply it to secondary batteries, it is natural that lithium is required to have high charging and discharging efficiency in the non-aqueous electrolyte. That is, the electrolytic solution used in a lithium secondary battery needs to simultaneously satisfy two requirements: high conductivity and high lithium charging/discharging efficiency. As an electrolyte with high charge/discharge efficiency for lithium,
A LiAsF ₆ -2-methyltetrahydrofuran-based electrolyte has been proposed (see US Pat. No. 4,118,550). ) However, the conductivity of this electrolyte was low (4.3×10 ^-3 Scm ^-1 for 1.5MLiAfF ₆ at 25°C), and when used in lithium batteries, it had the disadvantage of a low charge/discharge utilization rate. In addition, at low temperatures (~0℃), the complex of 2-methyltetrahydrofuran and Li ⁺ cannot be dissolved because the dielectric constant of 2-methyltetrahydrofuran is low (relative dielectric constant 6.2, 25℃), and in fact It also has the disadvantage that it becomes unusable. In order to eliminate such drawbacks, a mixed solvent in which 2-methyltetrahydrofuran and tetrahydrofuran are mixed has been proposed. However, as the mixing ratio of tetrahydrofuran increases, the conductivity of this mixed solvent improves;
There was a drawback that charging and discharging efficiency tended to decrease [P. Power Sources, Vol. 9, pp. 239-245 (1983)]. In addition, an electrolyte using ethylene carbonate as a solvent, for example, when using 1.5M LiAsF ₆ ,
LiAsF ₆ of 6.2×10 ^{-1 3} Scm ^-1 Although it exhibits about 45% higher conductivity than -2-methyltetrahydrofuran, it has the disadvantage that the charge and discharge efficiency of lithium is lower than that of 2-methyltetrahydrofuran [ Electorchimica Acta, Vol. 29, pp. 267-271 (1984)]. That is, to date, an electrolytic solution for lithium secondary batteries that has high conductivity and high lithium charging/discharging efficiency has not been realized. [Summary of the Invention] The present invention has been made in view of the current situation, and its purpose is to provide a lithium secondary battery with high electrical conductivity and excellent lithium charge/discharge characteristics. Therefore, the lithium secondary battery according to the present invention has the following characteristics:
The load active material is lithium or a lithium alloy that enables lithium ions to be discharged, the positive electrode active material is a material that electrochemically performs a reversible reaction with lithium ions, and the electrolyte is a lithium salt dissolved in an organic solvent. In a lithium secondary battery,
The organic solvent of the electrolytic solution is a mixed solvent of ethylene carbonate and 2-methyltetrahydrofuran with a volume mixing ratio of 1:1 to 1:9, and the electrolytic solution has a water content of 400 ppm or less and no impurities other than water.
It is characterized by being 25000ppm or less. According to the present invention, a mixed solvent of ethylene carbonate and 2-methyltetrahydrofuran with a volume mixing ratio of 50 to 90% is used as an electrolyte for a lithium secondary battery, and the amount of water and impurities other than water is controlled. The most important point is to realize a lithium secondary battery with high conductivity and charge/discharge efficiency. [Specific Description of the Invention] The present invention will be described in more detail. In a lithium secondary battery, the negative electrode active material is lithium or a lithium alloy that enables lithium ions to be discharged, the positive electrode active material is a material that electrochemically performs a reversible reaction with lithium ions, and the electrolyte is a material that electrochemically reacts with lithium ions. It is dissolved in a solvent,
According to the present invention, a mixed solvent in which ethylene carbonate and 2-methyltetrahydrofuran are mixed at a mixing ratio of 50 to 90% by volume is used as the organic solvent of the electrolytic solution in which a lithium salt is dissolved in an organic solvent. In order to increase the conductivity of the electrolyte used in lithium secondary batteries and the charging/discharging efficiency of Li, it is necessary to select a solvent with low reactivity for electron transfer from Li to the solvent, and to increase the dissociation of Li salt in the solvent system. Easy to use and Li ⁺
It is considered that high ion mobility is required. Ethylene carbonate has a high dielectric constant (relative permittivity 89.1, 40℃), but its melting point is about 36℃
It has the drawback that it is difficult to use alone at room temperature, and its viscosity is high (1.9 cP, 40
It also had the disadvantage of ℃). In order to compensate for the disadvantages of ethylene carbonate without impairing its advantages, 2-methyltetrahydrofuran is mixed in the present invention. The volume mixing ratio of ethylene carbonate and 2-methyltetrahydrofuran is 50 to 90%, preferably
60-70%, but 2 to ethylene carbonate
- Volume mixing ratio of methyltetrahydrofuran is 50
If it is less than 90%, there is not much difference from ethylene carbonate alone, whereas if it exceeds 90%, it becomes close to 2-methyltetrahydrofuran alone, and in either case, the improvement in charge/discharge efficiency and conductivity will not be sufficient. It is. The lithium salt dissolved in such a mixed solvent is not fundamentally limited in the present invention. For example, LiAsF ₆ , LiClO ₄ , LiBF ₄ , LiPF ₆ ,
One or more types of LiAlCl ₄ , LiCF ₃ SO ₃ , LiCF ₃ CO ₂ and the like can be effectively used. Such a lithium salt is added to the mixed solvent at a concentration of 0.5
It is preferable to add up to 1.5 mol/(M). Outside this range, not only will the conductivity decrease;
This is because there is a possibility that the charging/discharging efficiency of lithium may also be significantly reduced. The water content of such an electrolytic solution is determined in Example 1 described below.
As is clear from Table 1, it has become clear that the smaller the amount, the better the charge/discharge efficiency is. That is, when using the mixed solvent of ethylene carbonate and 2-methyltetrahydrofuran according to the present invention, the water content is preferably 400 ppm or less, preferably 50 ppm or less. When the water content exceeds 400ppm,
This is because the charging/discharging efficiency is significantly reduced. Further, as is also clear from Example 1 and Table 1, which will be described later, better charge/discharge efficiency can be obtained when the content of impurities other than water is also small. That is,
The content of impurities other than water is 25,000 ppm or less, preferably 1,000 ppm or less. This is because if the content of the impurities exceeds 25,000 ppm, the charge/discharge efficiency will be significantly impaired. The negative electrode active material used in the lithium secondary battery according to the present invention is basically not limited, and the negative electrode active material used in conventional lithium batteries, that is, lithium or a lithium alloy that can discharge lithium ions, is used. be able to. Similarly, the positive electrode active material used in the present invention is not fundamentally limited, and may be a positive electrode active material used in conventional lithium secondary batteries, that is, a material that electrochemically undergoes a reversible reaction with lithium ions. be able to. Among such positive electrode active materials, in the lithium secondary battery of the present invention, an amorphous material mainly composed of vanadium oxide such as V ₂ O ₅ , for example, V ₂ O ₅ alone, V ₂ O _{5 P2O5 , TeO2} , _{Sb2O3 ,}
Particularly preferred are materials to which one or more of Bi ₂ O ₃ , GeO ₂ , B ₂ O ₃ , MoO ₃ , WO ₃ and the like are added, but as mentioned above, the material is not limited thereto. Substances are used effectively. The above-mentioned amorphous material containing V ₂ O ₃ as a main component and adding V ₂ O ₅ can be obtained by melting and rapidly cooling a component to be mixed with V ₂ O ₃ , such as P ₂ O ₅ . . Examples will be described below. Example 1 1.5M LiAsF ₆ -ethylene carbonate/2-
Methyltetrahydrofuran (volume mixing ratio 1/
1) An electrolytic solution with controlled impurities was prepared, and a lithium secondary battery as described below was manufactured. The positive electrode contains 95 mol% V ₂ O ₅ −5 mol as an active material.
A positive electrode mixture pellet (16 mmφ) was prepared with a mixing ratio of 70% amorphous V ₂ O ₅ with a composition of %P ₂ O ₅ , 25% by weight of acetylene black as a conductive agent, and 5% by weight of Teflon as a binder. Metal lithium (10mmφ, 30mAh or 17mmφ,
A coin-type lithium battery was manufactured using a microporous polypropylene sheet as a separator. This lithium battery has a current value of 1 mA at room temperature,
A charge/discharge test was conducted in a voltage range of 2 to 3.5 V to evaluate the charge/discharge characteristics of the electrolyte. The results are shown in Figure 1. As is clear from Figure 1, after 10 cycles at a constant capacity, the excess lithium that can participate in discharge is consumed on the negative electrode side, and the discharge capacity gradually decreases according to the charge/discharge efficiency E. I will continue to do so. This decrease in discharge capacity is not due to the positive electrode active material, as the capacity becomes approximately constant when lithium is present in excess, as will be clear from Example 9 below. In other words, if E is the charging/discharging efficiency of the negative electrode and Cn is the discharge capacity of the nth party, then Cn=E−Cn _-1 holds, and from this, the relationship InCn=(n−1)lnE+lnE+lnC ₁ can be found, and the charging Discharge efficiency can be calculated. The charge/discharge efficiency was calculated using the above formula by changing the amount of water and impurities in the electrolyte using the mixed solvent of ethylene carbonate: 2-methyltetrahydrofuran = 1:1.
Shown in the table. In Table 1, when comparing electrolyte 1, electrolyte 3, electrolyte 4, and electrolyte 5, it can be seen that the lower the water content of the electrolyte, the higher the value of E becomes.
Further, when comparing electrolytic solution 2 and electrolytic solution 4 in Table 1, it was found that the value of E increases as the content of impurities other than water decreases. It is clear from Table 1 that the charge/discharge efficiency is significantly improved by removing water and other impurities. According to this, it was found that especially high charge/discharge efficiency can be obtained when the water content of the electrolytic solution is 400 ppm or less, preferably 50 ppm or less, and the impurities other than water are 25000 ppm or less, preferably 1000 ppm or less. In addition, when comparing electrolyte 6 and electrolyte 5, it is reported that electrolyte 2 is effective in improving charging and discharging efficiency.
-Methylfuran (MeF) [J.Electorochem.Soc.,
131, p. 2197, (1984)] was not found. In the examples shown below, unless otherwise specified, an electrolytic solution having a composition very close to that of the electrolytic solution 4 is used as the electrolytic solution.

[Table] Example 2 A mixed solvent of ethylene carbonate and 2-methyltetrahydrofuran containing 1.5M of electrolyte.
A solution of LiAsF ₆ was used. FIG. 2 shows the relationship between the conductivity (K) of this electrolytic solution, () the amount of 2-methyltetrahydrofuran mixed, and () the temperature.
It was found that a mixture of ethylene carbonate and 2-methyltetrahydrofuran showed higher conductivity than a single solvent electrolyte. For example, 25℃
When the volume mixing ratio of ethylene carbonate/2-methyltetrahydrofuran was 2/3, maximum electrical conductivity was obtained that was 1.6 times and 2.3 times that of ethylene carbonate and 2-methyltetrahydrofuran single solvent electrolytes, respectively. It was done.
Even on the low temperature side, the ethylene carbonate/2-methyltetrahydrofuran mixed solvent system showed higher values than the 2-methyltetrahydrofuran alone system. 2-Methyltetrahydrofuran has a much stronger solvation power for Li ⁺ than ethylene carbonate, and Li ⁺ almost selectively acts as a solvent for 2-methyltetrahydrofuran even in the ethylene carbonate/2-methyltetrahydrofuran mixed system. It is harmonized. In the case of 2-methyltetrahydrofuran alone, the complex of 2-methyltetrahydrofuran and Li ⁺ cannot be dissolved at low temperatures, but ethylene carbonate/
Since the dielectric constant is improved by mixing 2-methyltetrahydrofuran, it is thought that this negative effect on the mobility of Li ⁺ is eliminated (the dielectric constant of the mixed solvent is almost linear with the volume mixing ratio). ). It was also found that on the low temperature side, the amount of 2-methyltetrahydrofuran mixed, which exhibits maximum electrical conductivity, changes to around 70%. From the conductivity characteristics, the volumetric mixing amount of 2-methyltetrahydrofuran is 50 to 90%,
It has been found that 60 to 70% is particularly preferable. Example 3 Figure 3 shows LiAsF ₆ -ethylene carbonate/2
-Methyltetrahydrofuran (3/7) and
The relationship between LiAsF ₆ concentration is shown. As a result, it was found that good conductivity was exhibited at a LiAsF ₆ concentration of 0.5 to 2.0M. In particular, 1.0 to 1.5M showed high values. Of course, as shown in Figure 3, compared to 2-methyltetrahydrofuran alone,
Ethylene carbonate/2- in the range of 0.5-2.0M
The methyltetrahydrofuran mixed system showed high conductivity. Since the dielectric constant increases by mixing ethylene carbonate and 2-methyltetrahydrofuran, the ion association constant (KA) becomes smaller than that of 2-methyltetrahydrofuran alone (the degree of ion dissociation increases). Table 2 shows KA calculated from Bjerrum's formula. Also, when 2-methyltetrahydrofuran is mixed with ethylene carbonate, the viscosity decreases,
The Li ⁺ ion conductivity is improved compared to ethylene carbonate alone (the viscosity of the mixed solvent changes hyperbolically with the volumetric mixing ratio). At infinite dilution, the transference number (t ₀ ⁺ ) and ultimate equivalent conductivity (Λ ₀ ) were measured, and Li ⁺ ion conductivity (Λ ₀ ⁺ =Λ ₀ ×t ₀ ⁺ ) was determined. The results are shown in Table 3. The calculation of KA and the measurement of Λ ₀ ⁺ were carried out by the method described in the reference [Electorochimica.Acta., Vol. 29, pp. 1471-1476 (1984)]. In other words, the characteristics of the ethylene carbonate/2-methyltetrahydrofuran mixed system are that it has a higher degree of ion dissociation than the 2-methyltetrahydrofuran system alone, and has higher Li ⁺ ion conductivity than the ethylene carbonate system alone; higher than that of carbonate and 2-methyltetrahydrofuran alone. Furthermore, even when the solute concentration and temperature are changed, the electrolyte exhibits higher conductivity than the single electrolyte, and the ethylene carbonate/2-methyltetrahydrofuran mixed electrolyte exhibits extremely excellent conductivity characteristics. In particular, it was found that good characteristics were exhibited when the volumetric mixing amount of 2-methyltetrahydrofuran was 60 to 70%.

【table】

[Table] Example 4 The charging and discharging efficiency of lithium was measured using 1.5M LiAsF ₆ -ethylene carbonate/tetrahydrofuran as the electrolyte. Charge/discharge efficiency (Ea)
A battery was assembled with a platinum electrode as the working electrode, lithium as the counter electrode, and lithium as the reference electrode, and measurements were performed as follows. In the measurement, first, lithium was deposited on a platinum electrode (2.4C/cm ² ) for 80 minutes at a constant current of 0.5 mA/cm ² , and then a portion of the deposited lithium (0.6C/cm ² ) was deposited on the platinum electrode. A cycle test was repeated in which the battery was discharged as Li ⁺ ions and then further discharged at a capacity of 0.6 C/cm ² . The charging/discharging efficiency (Ea) is determined from the change in the potential of the platinum electrode, and if the number of cycles showing an apparent 100% efficiency is n, then the Ea can be determined from the following formula (). Ea=[0.6−2.4−0.6/n/0.6]×100(%)……(
) Figure 4 shows the relationship between the Ea of 1.5M LiAsF ₆ -ethylene carbonate/2-methyltetrahydrofuran and the mixing amount of 2-methyltetrahydrofuran. The Ea of the mixed system was higher than that of ethylene carbonate and 2-methyltetrahydrofuran alone, and particularly when the volume of ethylene carbonate mixed was 50 to 90%, excellent characteristics were exhibited. The causes of the decrease in lithium charge/discharge efficiency are () electrochemical inactivation of lithium due to the reaction between the precipitated lithium and the solvent, and () poor smoothness of the lithium precipitated form. It is conceivable that the lithium becomes crystalline (supercrystalline) and peels off or falls off from the lithium electrode. Regarding the smoothness of metal deposition, the precipitation of Zn in alkaline aqueous solutions has been widely studied, and the deposition form improves the mass transfer of ions involved in the reaction in the electrolyte and improves the precipitation reaction mechanism. (e.g. additives) [Advances in
Electrochemistry and Electrochemistry
Engineering: Volume 11, p. 273 (1978)]. It is believed that this invention can also be applied to the precipitation of Li metal,
It is expected that the above methods can improve the smoothness of the precipitation form of ()Li. Mixing ethylene carbonate and 2-methyltetrahydrofuran is expected to effectively address the problems of () and () above. Regarding the inactivation of lithium () above, Li ⁺ resulting from the precipitation reaction is mainly solvated by 2-methyltetrahydrofuran, and 2-methyltetrahydrofuran has electrons on the oxygen atom that participate in the reaction with lithium. Because of its high density, it is difficult to be reduced by lithium, and the concentration of 2-methyltetrahydrofuran becomes high near the precipitated lithium, and a solvent atmosphere with low reactivity toward lithium is formed. Therefore, the reaction between the precipitated lithium and the solvent is suppressed. The smoothness of the deposited form of lithium () is thought to be due to the fact that the addition of ethylene carbonate facilitated the mass transfer of ions. In other words, the degree of dissociation of LiAsF ₆ has improved and the conductivity has improved, so the supply of Li ⁺ ions has become smoother, and the state of precipitation on the Li surface has been improved due to the above reasons (improved ion mobility). it is conceivable that. The reason why the Ea of the ethylene carbonate/2-methyltetrahydrofuran mixed system is higher than the Ea of the 2-methyltetrahydrofuran alone system is thought to be due to the reason () above. Example 5 Figure 5 shows LiAsF ₆ -ethylene carbonate/2
- in methyltetrahydrofuran (3/7)
The relationship between Ea and LiAsF ₆ concentration is shown. In particular, 1.0~
It was found that good values were shown at 1.5M. Example 6 1.5M LiAsF ₆ -ethylene carbonate/2-methyltetrahydrofuran (3/7) was used as the electrolyte, and Li/Al (atomic ratio 9/1) alloy (Li electrochemical Discharge capacity is 10m
A battery was fabricated using a Li plate (200 mAh) as a counter electrode, and Li as a reference electrode.
A cycle test was conducted in which the battery was charged and discharged for hours. Table 4 shows the charging and discharging efficiency of Li obtained through this measurement. As a reference example, Table 4 also shows the results of the same measurement using 1.5M LiAsF ₆ -2-methyltetrahydrofuran as the electrolyte. By using the ethylene carbonate/2-methyltetrahydrofuran mixture as a solvent for the electrolytic solution, a higher charging/discharging efficiency of LiAsF ₆ was obtained than that of 2-methyltetrahydrofuran alone.

[Table] Example 7 1.5M LiAsF ₆ -ethylene carbonate/2-
Using methyltetrahydrofuran, the positive electrode was
A mixture of 70% by weight of amorphous V ₂ O ₅ with a composition of 95mol%V ₂ O ₅ -5mol%P ₂ O ₅ as an active material, 25% by weight of acetylene black as a conductive agent, and 5% by weight of Teflon as a binder. Using a positive electrode mixture pellet (16 mmφ) made with a ratio of
A coin-type lithium battery (23mmφ, 2mm thickness) was manufactured. These coin type batteries are 1, 2, 4, 8,
The discharge capacity and 2-
The relationship with the amount of methyltetrahydrofuran mixed was determined. The results are shown in Figure 6. From Figure 6, it can be seen that the ethylene carbonate/2-methyltetrahydrofuran mixed system in which 2-methyltetrahydrofuran is mixed at a volume mixing ratio of 50 to 90% has a higher discharge capacity than the 2-methyltetrahydrofuran alone system. You can see that it's big. This result corresponds well to the trend shown in FIG. 2, and the discharge capacity tends to be large in electrolytic solution systems exhibiting high conductivity. In particular, it was found that good characteristics were exhibited when the volumetric mixing ratio of 2-methyltetrahydrofuran was 60 to 70% at a second current of 8.12 mA. Example 8 A coin-type battery was produced in the same manner as in Example 7,
At room temperature, with a discharge current of 6 mA and a charging current of 2 mA,
Charge/discharge tests were conducted in a voltage range of 3.5V. The amount of Li in the battery was 90mAh. Figure 7 shows 1.5M LiAsF ₆ -ethylene carbonate/2-methyltetrahydrofuran (1/
The results are shown for the mixed system of 1) and for the 1.5M LiAsF ₆ -2-methyltetrahydrofuran alone system. In this battery, the amount of positive electrode active material was 84 mg in the battery using ethylene carbonate/2-methyltetrahydrofuran, and 97 mg in the battery using 2-methyltetrahydrofuran alone. In the case of ethylene carbonate/2-methyltetrahydrofuran (1/1), the discharge capacity per cycle is generally larger than that of 2-methyltetrahydrofuran alone, and the capacity is 1/2 of the initial capacity. The number of cycles required to achieve this is 250, far exceeding the 120 cycles for 2-methyltetrahydrofuran.
The cause of the decrease in capacity is thought to be an increase in internal resistance of the battery, as the open circuit voltage after discharge increases as the number of cycles increases, and even if the battery is made again using the positive electrode plate after the cycle, the characteristics will not deteriorate. Since this is not observed, it is thought that the increase in resistance is caused by the Li electrode side. The Li charging/discharging efficiency is calculated from the number of cycles until the battery capacity reaches 3 mAh and the amount of Li remaining in the cathode, and it is 95.7% (1/1-E=
23), it was 87.0% (1/1-E=8) in the 2-methyltetrahydrofuran-only cell. Example 9 Metallic lithium of the negative electrode has an electrical capacity of 30 mAh (10
Example 8
A coin-type battery was prepared in the same manner as above, and it was heated to 1 at room temperature.
A charge/discharge test was conducted at a current value of mA and a voltage range of 2 to 3.5 V, and the charge/discharge characteristics of the electrolytic solution were evaluated in the same manner as in Example 1. The results are shown in Figure 8. Ethylene carbonate/2-methyltetrahydrofuran mixed systems have higher charge/discharge efficiency than 2-methyltetrahydrofuran alone, especially when the volume ratio of 2-methyltetrahydrofuran is 60 to 80. In this case, the efficiency was higher than that of a 50% mixed electrolyte system. This result is consistent with the effect in the case of a lithium monopole. Example 10 Using 19mmφ positive electrode mixture pellets, 16mmφ, 90
Using mAh metallic lithium, 1.5M liAsF ₆ −
A coin-type battery (23 mmφ, 2 mm thickness ) was created. At room temperature, current value of 2 mA, 2
A charge/discharge test was conducted in a voltage range of ~3.5V to evaluate battery characteristics. The results are shown in Figure 9. An initial capacity of 400 mAh was obtained, and the energy density was high at 48.1 mA/cc and 125 Wh/. Since the difference between the first discharge capacity and charge capacity is the amount of Li that is not removed from the positive electrode due to charging, the first
The amount of Li used up to 20 times is 90−35−32=23m
Ah. Using this value and the cumulative charging capacity up to the 20th charge, the lithium charge/discharge efficiency is 97.0%
(1/1-E=23.4). Example 11 As positive electrode active material, 80molV ₂ O ₅ −20mol%
A coin-type battery was produced in the same manner as in Example 7 except that MoO ₃ and 80 mol% V ₂ O ₅ -20 mol% WO ₃ were used, and a charge/discharge test was conducted under the conditions of 1 mA; 140 Ah/Kg. Table 5 shows the results when 1.5M LiAsF ₆ -ethylene carbonate/2-methyltetrahydrofuran was used as the electrolyte in comparison with the results when 1.5M LiAsF ₆ -2-methyltetrahydrofuran was used. As can be seen from Table 5, the characteristics are improved by using the mixed system.

〔Effect of the invention〕

As explained above, the lithium secondary battery according to the present invention has the advantage that it is a small, high energy density battery with a large charge/discharge capacity and excellent cycle life, and can be widely used in various fields. .

[Brief explanation of the drawing]

Figure 1 shows 1.5M LiAsF ₆ -ethylene carbonate/2-methyltetrahydrofuran (1/1)
1mA of battery using 30mAh Li in the system, 2~
Figure 2 shows the change in discharge capacity of cycle paper in the 3.5V range. Figure 2 shows the conductivity of 1.5M LiAsF ₆ -ethylene carbonate/2-methyltetrahydrofuran. Figure 3 shows the conductivity of 1.5M LiAsF 6 -ethylene carbonate/2-methyltetrahydrofuran. A diagram showing the relationship between LiAsF ₆ concentration and conductivity in gas sensor (3/7) and 2-methyltetrahydrofuran. Figure 4 is 1.5M LiAsF ₆
- Figure 5 shows the relationship between the amount of 2-methyltetrahydrofuran mixed and the charge/discharge efficiency Ea in the ethylene carbonate/ ₂ -methyltetrahydrofuran mixed system. MeTHF
Figure 6 shows the relationship between LiAsF ₆ concentration and Ea in (3/7), and the amount of 2-methyltetrahydrofuran mixed in the LiAsF ₆ -ethylene carbonate/2-methyltetrahydrofuran mixed system and each current value. Figure 7 shows the relationship between discharge capacity and discharge capacity of 2mA, 2~
Figure 8 shows the change in discharge capacity for each cycle when performing a charge/discharge test in the 3.5V range.
Figure 9 shows the relationship between the amount of 2-methyltetrahydrofuran mixed and the charging/discharging efficiency of the battery in the 1.5LiAsF ₆ -ethylene carbonate/2-methyltetrahydrofuran system. Figure 9 shows the relationship between the battery capacity and the number of charging/discharging cycles. A diagram showing the relationship, FIG. 10, is a diagram showing the relationship between battery capacity and the number of charge/discharge cycles.

Claims (1)

[Claims] 1. The load active material is lithium or a lithium alloy that makes it possible to discharge lithium ions, the positive electrode active material is a material that electrochemically undergoes a reversible reaction with lithium ions, and the electrolyte contains lithium salts. In a lithium secondary battery that is dissolved in an organic solvent, the organic solvent of the electrolyte is a mixed solvent of ethylene carbonate and 2-methyltetrahydrofuran in a volume mixing ratio of 1:1 to 1:9; The water content of the electrolyte is 400ppm or less, and there are no impurities other than water.
A lithium secondary battery characterized by a content of 25,000ppm or less. 2 The electrolyte contains LiAsF ₆ , LiClO ₄ , LiBF ₄ ,
0.5% of one or more lithium salts selected from the group consisting of LiPF ₆ , LiAlCl ₄ , LiCF ₃ SO ₃ , and LiCF ₃ CO ₂
1.5 mol/dissolved lithium secondary battery according to claim 1. 3 The positive electrode active material is V ₂ O ₅ alone or in combination with V ₂ O _5
P2O5 , _TeO2 , _{SbO3 , Bi2O3 , GeO2} , _{B2O3 ,}
The lithium secondary battery according to claim 1 or 2, characterized in that it is an amorphous material to which one or more of P ₂ O ₅ , MoO ₃ , and WO ₃ is added.